Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly (unit cell) is interposed between separators (bipolar plates). In use, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
As this type of the fuel cell, for example, a solid oxide fuel cell as disclosed in Japanese Laid-Open Patent Publication No. 3-129675 (hereinafter referred to as the first conventional example) is known. As shown in FIG. 35, the fuel cell is formed by stacking donut shape separators 1 and donut shape unit cells 2 alternately. A fuel gas inlet pipe 3a and an oxygen-containing gas inlet pipe 3b as manifolds extend through the centers of the separators 1 and the unit cells 2. In the outer circumferential portion of the fuel cell, fuel gas exhaust ports 4a and oxygen-containing gas exhaust ports 4b are provided offset from each other at an angle of 90°.
Further, in a solid oxide fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2002-8682 (hereinafter referred to as the second conventional example), as shown in FIG. 36, circular disk shape solid electrolyte plates (not shown) and circular disk shape separators 5 are stacked alternately. In the outer circumferential portion of the separator 5, a fuel gas supply hole 6a and an oxygen-containing gas supply hole 6b are provided offset from each other at an angle of 180°, and a plurality of fuel gas exhaust nozzles 7 and oxygen-containing gas exhaust nozzles (not shown) are provided at predetermined intervals in the circumferential direction.
In the surface of the separator 5, seven recesses 8 are provided, and annular fuel gas pipes 9 connecting the seven recesses 8 are provided. The pipes 9 and the fuel gas supply holes 6a are connected to each other. In the surface of the separator 5, spiral fuel gas grooves 10 are provided around the recesses 8.
Further, in a stack unit assembly disclosed in 2005 Fuel Cell Seminar. Nov. 14-18, 2005. Palm Springs, Calif. “Development of High-Efficiency SOFC Module” (hereinafter referred to as the third conventional example), as shown in FIG. 37, a square separator 12 is provided. The separator 12 includes three plates 14a, 14b, and 14c. A fuel manifold 15a and an air manifold 15b extend through the separator 12 at diagonal positions. The fuel manifold 15a and the air manifold 15b are provided at ends of flexible arms 16a, 16b where slits are formed.
A fuel channel 17a and an air channel 17b are provided spirally on the plate 14b. A fuel outlet 18a connected to the fuel channel 17a is provided on the plate 14a, and an air outlet 18b connected to the air channel 17b is provided on the plate 14c. 
However, in the first conventional example, the fuel gas exhaust ports 4a and the oxygen-containing gas exhaust ports 4b are provided in the outer circumferential portion of the fuel cell, and the fuel gas and the oxygen-containing gas after reaction (hereinafter also referred to as the exhaust gas) are only discharged from the fuel gas exhaust ports 4a and the oxygen-containing gas exhaust ports 4b. In the structure, the exhaust gas from the unit cells 2 may be discharged non-uniformly or locally, or filled or stayed in local spots. Therefore, power generation is not performed efficiently, and the power generation output by the unit cells 2 may be lowered undesirably.
Further, the fuel gas inlet pipe 3a and the oxygen-containing gas inlet pipe 3b as the manifolds are provided integrally with the separators 1 at positions near the central axis of the separators 1. In the structure, when a tightening load is applied to the unit cells 2 and the separators 1 in the stacking direction to achieve the desired sealing performance of the fuel gas and the oxygen-containing gas, the unit cells 2 may be damaged undesirably by the excessive stress due to the tightening load.
Further, in the second conventional example, the fuel gas (exhaust gas) after reaction is only discharged from the fuel gas exhaust nozzles 7 provided in the outer circumferential portion of the separators 5. In the structure, the exhaust gas from the separators 5 may be discharged non-uniformly or locally, or filled or stayed in local spots. Therefore, power generation is not performed efficiently, and the power generation output by the unit cells may be lowered undesirably. Further, in the structure, when a tightening load is applied to the unit cells and the separators 5 in the stacking direction to achieve the desired sealing performance for preventing the leakage of the fuel gas and the oxygen-containing gas, the unit cells may be damaged undesirably by the excessive stress due to the tightening load.
Further, in the third conventional example, the flexible arms 16a, 16b having slits in the outer circumferential portion of the separator 12 are provided. The fuel manifold 15a and the air manifold 15b are connected to the spiral fuel channel 17a and air channel 17b. In the structure, when a tightening load is applied to the separator 12 in the stacking direction to achieve the desired sealing performance for preventing the leakage of the fuel and the air, the flow of the exhaust gas from the outer circumferential portion of the electrolyte electrode assembly is prevented by the flexible arms 16a, 16b. Thus, exhaust gas is discharged non-uniformly or locally, or filled or stayed in local spots. Therefore, power generation is not performed efficiently, and the power generation output is lowered undesirably.
Further, the separator 12 has a square shape, and the electrolyte electrode assembly has a circular shape. Thus, the area occupied by the separator 12 is large relative to the reaction area of the electrolyte electrode assembly. Therefore, the electricity collection efficiency per unit area and the space efficiency are low.